A large and growing market exists for analytical devices which can be used to analyze the reaction products of a wide variety of industrial processes. In addition, on-line real time combustion product analyzers are needed to facilitate more efficient burning of hydrocarbon fuel. For example, the electric utility has been faced with the task of monitoring stack emissions for some years. At first, this consisted of measuring the oxygen concentration for controlling combustion efficiency, and of monitoring smoke opacity. Further regulations have led to the necessity of monitoring sulfur oxides and nitrogen oxides. Additional gases will undoubtedly also require monitoring in the future. Proposed regulations in Europe, for example, will require the monitoring of ammonia if it is added to the stack to maintain precipitron efficiency. Recent studies have shown that increased combustion efficiency can be achieved if carbon monoxide is monitored. The list of gases is rapidly growing to larger and larger numbers. The historical solution has been to install a separate sensor to monitor each gas. This approach becomes more and more expensive and a less desirable approach as the total number of gases for which monitoring is required increases.
It is known to employ analytical devices which utilize ultraviolet and infra-red spectrophotometry, as well as gas and liquid chromatography in order to meet the industrial needs outlined above. Such optical instruments as well as the system of the present invention utilize the following important characteristics of materials. A particular molecule has a characteristic absorption spectrum which is dissimilar to that of all other molecules. The spectra of mixtures of molecules are additive and the absorption is proportional to the concentrations of the molecules. Optical absorption spectra can be obtained from any type of sample be it solid, liquid, or gas so long as the sample is optically transmissive, and the spectra can be obtained in a non-destructive testing of the sample. Almost all gas molecules absorb infra-red radiation. Each molecule has its absorption at specific characteristic wavelengths, so that by monitoring the magnitude of the absorption at the specific wavelengths, the amount of gas present in the stack may be deduced. This is accomplished by transmitting infra-red radiation across the stack to a detector. The wavelength of the transmitted radiation is tuned to coincide with the specific wavelength of the gas to be measured. The strength of the detector signal at a given wavelength correlates with the concentration of the absorbing gas at that wavelength. The selective tuning of the infra-red radiation is accomplished either by inserting narrow band interference filters in the path, or by inserting cells containing appropriate gas fills in the path. Both of these approaches involve mechanical motion, and the number of cells or filters that can be employed is obviously limited. Thus reliability and accuracy suffer from the less than ideal properties of these tuning elements.
It has recently been recognized that certain birefringent optical materials which are termed acousto-optic materials can be used as a filter in a spectrum analyzer. In such acousto-optic materials, a light beam propagating as an E-ray can under certain conditions be converted into an O-ray by interaction with, and diffraction from, an acoustic wave propagating from the same medium. This phenomenon has been used in fabricating narrow band optical filters, the peak transmission wavelength of which can be selected by properly choosing the frequency of the acoustic wave. Even more recently, new efficient infra-red transmissive acousto-optic materials such as thallium-arsenic-selenide, as described in U.S. Pat. No. 3,792,287 which is owned by the assignee of the present invention and incorporated herein by reference, provide the possibility of operation over the near to mid-infra-red spectrum from about 1 micrometer to about 16 micrometers.
An automated acousto-optic infra-red analyzer system which utilizes acousto-optic technology is described in U.S. Pat. No. 4,490,845 which is assigned to the assignee of the present invention and incorporated herein by reference as it fully set forth. U.S. Pat. No. 4,490,845 teaches an automated acousto-optic tunable filter infra-red analyzer system which permits rapid electronic tuning of the filter to a selected infra-red bandpass via the acousto-optic interaction with infra-red radiation which has passed through a sample. The infra-red analyzer system includes means for directing infra-red radiation through the sample to be analyzed, which sample has a predetermined infra-red absorption characteristic. Means are provided for directing the infra-red radiation through an acousto-optic tunable filter after the infra-red radiation has passed through the sample species. An acousto-optic tunable filter includes an input polarizer for selectively polarizing the infra-red radiation. The tunable filter includes an optically aligned acousto-optic crystal through which the selectively polarized infra-red radiation is passed at a predetermined angle relative to the crystal optic axis. An acoustic transducer means is coupled to the crystal and to a variable frequency RF energy source whereby acoustic waves are launched in the crystal in order to interact with the selected narrow bandwidth portion of the polarized infra-red radiation to make it distinguishable from the remaining radiation. The tuned or selected narrow bandwidth radiation is a function of the frequency of the RF energy source which is connected to the acoustic transducer of the filter. Infra-red radiation detection means are coupled to the filter to detect the output filtered infra-red radiation and to generate an output signal as a function of the output filtered infra-red radiation. Automated computing means are provided, with the detection means output electrical signal applied to the computing means for determining the sample species present in the sample. The computing means includes means for selectively activating the RF energy source to determine the timing and frequency of RF energy applied to the acoustic transducer to thereby determine the selected or filtered narrow bandwidth infra-red wavelength of interest.
It is an object of the present invention to provide an improved automated acousto-optic infra-red analyzer system which is particularly well suited for the task of monitoring stack gas emissions. The improved analyzer system is an electronically operated device that easily interfaces with a microcomputer for "smart sensor" applications such as rapid scan differential absorption spectroscopy, signal conditioning, and comparison of measured absorption spectra with stored spectra.
It is a further object of this invention to provide an improved automated acousto-optic infra-red analyzer system in which the infra-red detector and the acousto-optic tunable filter are separated so that the angular displacement of the narrow-band interacted radiation is adequate to separate it spatially from the broad-band non-interacted radiation at the detector, thus eliminating the need for crossed polarizers.
It is yet another object of this invention to provide an improved automated acousto-optic infra-red analyzer system with a configuration in which the AOTF is transmitting narrow-band pulsed or chopped radiation across the gas containing stack to the detector. Thus, the detector can discriminate between these pulsed emissions and the steady thermal emissions from the hot stack. Accordingly, the improved analyzer system is particularly well suited for use in applications in which the gas to be analyzed is of a very high temperature.
It is yet another object of this invention to provide an analyzer system which has the potential for inexpensive manufacture and is a solid state device having no moving parts and therefore a very high level of reliability.
It is still another object of this invention to provide an improved automated acousto-optic infra-red analyzer system capable of infinitely variable wavelength selection which can be used to measure any required gas within an environment of interest.
It is still another object of this invention to provide an improved automated acousto-optic infra-red analyzer system which is capable of covering the entire infra-red region which is useful for measuring gas absorptions.